In order to maintain the output performance of an internal combustion engine at a high level over a broad range of the rotation speed of the engine and to allow the engine to continuously exert a high output, intake and exhaust valves are required to have such valve operation characteristics that the lifting quantities of the valves are small and the valve opening-closing timing ranges are narrow in a low rotation speed region or low load region of the engine whereas the lifting quantities of the valves are large and the valve opening-closing timing ranges are broad in a high rotation speed region or high load region of the engine. However, according to conventional valve-operating mechanisms, since means for adjusting operation characteristics of valves in conformity with changes in the rotation speed of an engine are not provided at all, intake and exhaust valves are caused to make opening and closing operations always in fixed manners, and therefore, it is difficult to ensure such valve operation capacities as will allow the engine to exert a sufficient output performance over a broad range extending either from the low rotation speed to the high rotation speed or from the low load region to the high load region. In other words, according to the uses and application conditions of an internal combustion engine, the performance is inevitably reduced in either of the high and low rotation speed regions or either of the high and low load regions.
In conventional intake and exhaust valve operating mechanisms, it is known to eliminate variations or changes in the valve operation caused by vibration or abrasion of intake and exhaust valves, by feeding an oil into a valve lifting means (hereinafter referred to as "valve lifter") from an oil pump to actuate the valve lifter to compensate for the variations or changes. More specifically, the clearance in the valve lifter is diminished by the structure in which the valve lifter is operated by utilizing the pressure of the oil, whereby vibrations of intake and exhaust valves are prevented and the clearance in the intake and exhaust valve lifting mechanism formed by abrasion of the intake and exhaust valves is eliminated in the interior of the lifter, thus resulting in rectifying variations in the valve operation.